Clean up magic_methcall docs

Clean up magic_methcall docs

This is rather unsightly, don’t you think?

magic_methcall
        Invoke a magic method (like FETCH).
        * sv and mg are the tied thingy and the tie magic; * meth is
        the name of the method to call; * argc is the number of args
        (in addition to $self) to pass to the method;
               the args themselves are any values following the argc
        argument.  * flags:
            G_DISCARD:     invoke method with G_DISCARD flag and don’t
        return a value
            G_UNDEF_FILL:  fill the stack with argc pointers to
        PL_sv_undef.
        Returns the SV (if any) returned by the method, or NULL on
        failure.

(That’s the ‘rendered’ nroff output.)

I would have used =over/=item/=back, but autodoc.pl doesn’t seem to
like those.